[vladmoz]

What you describe -- "imagine every one having to go through a DOM interface and every button and UI interaction being composed of DOM elements and re-styling these elements on every interaction" -- is exactly how Firefox is built. The desktop Firefox UI is all DOM elements (not HTML, but still the same underlying DOM code).

Hovering over a toolbar button or a tab causes a CSS style rule for a :hover effect to be applied. Opening a new tab uses CSS animation for the transition. Moving tabs around manipulates the DOM and moves the tab elements (which are themselves composed of images, labels, etc.).

And yet, the Firefox UI is plenty snappy -- yes, you can argue that there are slowdowns and issues that need to be fixed, but no more or less than in many "native" apps.